Magnetic pigments have been used in electronic recording devices since the late 1940's. The moderate cost of production and the chemical stability maghemite the principal magnetic pigment for the purpose. Though magnetite, another iron oxide, displays high magnetization and coercivity, it is less suitable for recording devices due to its magnetic instability.
Much of the art of creating a good magnetic medium lies in the preparation method of the pigment. New materials for magnetic recording may be limited by the superparamagnetic relaxation produced by the small size of the particle (<30 nm). Particle shape is also an important factor as it determines the shape anisotropy and then the coercive field value. For longitudinal magnetic recording, particles must show acicular shape.
The conventional route of synthesis of acicular maghemite particles through oxyhydroxide method is multistep and complicated. It becomes further difficult to produce nanosized maghemite particles through this process as a result of a poor control over the growth kinetics/in another method, oxidation of magnetite particles result in formation of either cubic or irregular maghemite particles. The crystals of <300 nm size are completely transformed to the maghemite phase at 200–250° C. Whereas, for larger crystals, the oxidation is incomplete. The process being single step, however, involves a relatively higher temperature to produce maghemite particles of negligible aspect ratio.
Reference may be made to R. Robl, Anges Chem. 1958, 70. pp. 367, wherein oxidation of ferrous sulphate solution was earned out with potassium nitrate followed by which the solution was heated to 60–80° C. and added sodium hydroxide solution slowly with continuous stirring. The black precipitate formed was washed and dried and was heated to 250° C. for 30 minutes leading to the formation of maghemite particles.
Reference may be made to R. M. Taylor and U. Schwertman, Clay Min. 1974; 10, pp. 299–310, wherein equal volumes of ferrous and ferric salt solutions were mixed and was heated to 40° C. To this was added the requisite amount of sodium hydroxide when a black precipitate was formed. This was filtered, washed, dried in air when the precipitate turned dark brown and was identified to be maghemite.
Reference may be made to Y. Maeda, The Electronics & Tele-Communication laboratories, NNT, E:C.L Techn. Publ., 1978, 179, pp 1–7, wherein-a solution of ferrous sulphate with sodium hydroxide leads to the formation of ferrous hydroxide. This was oxidized rapidly by passing air for 45 minutes with stirring which results in the formation of ferrous oxyhydroxide. This was again mixed with a solution of ferrous sulphate and iron wire was placed in the solution followed by which 8–10 litre air/minute was passed for 48 hours while maintaining the temperature at 60° C. The ferrous oxyhydroxide was separated from the solution, washed and dried at 130° C. and heated with hydrogen in a stirred autoclave at 440° C. until the magnetite content was 23.8%. After cooling to 250° C. air was passed in until the ferrous salt was absent.
Reference may be made to R. D. Gaulam and Madan Rao. Mater. Res. Bull. 1982, 17, pp. 443, wherein lepidocrocite is treated with pyridine and carefully oxidized for a long period for complete conversion to nanosized maghemite particles.
Reference may be made to G. Ennas, G. Marongm. A. Musinu. AiFalqw.-P. BalUrano and Camintin. J. Mater Res. 1999,14, pp.1570, wherein, through a wet chemical synthesis of successive hydrolysis, oxidation and dehydration of ferrous chloride was performed to obtain maghemite particles as small as 5 nm.
Reference may be made to G. V Gopal Reddy. Sheela Kalvana anil S. V Manorama, hit. J. Inorg. Mater, 2000, 2; pp.301, wherein synthesis of maghemite for sensor application through a novel technique of combustion of ferric salts with hydrazine hydrate has been carried out.
Reference may be made to K. E. Gonsalves, H. Li, and P. Santiago, J. Mattr. ScL 2001,36, pp. 2461, wherein lepidocrocite has been converted into maghemite by colloidal process in which the particles could be readily dispersed into an organic solvent. The as-prepared-acicular maghemite nanorod-ethanol dispersion containing 0.005 g nanorods was centrifuged. The supernatant solvent was decanted followed by the addition of 8.3 g of 6 wt % PMMA (polymethyl methacrylate) solution, and the mixture was sonicated for several hours in an ice-water cooling bath. The concentration of the magnetic maghemite nanorods was about 1% relative to the PMMA.
All the above processes are expensive and involve complicated steps.
Moreover, the maghemite particles produced have poor crystallinity mixed phase. Random variation in morphology low aspect ratio and magnetically induced agglomeration. The above limitations reduce the applicability the magnetic pigments in the field of magnetic information storage.
Reference may be made to Wolfgang H. H. Gunther et al. U.S. Pat. No. 6,123,920. dated Sep. 26, 2000, entitled “Superparamagnetic contrast media coated with starch and polyalkylene oxides wherein MR contrast media containing composite nanoparticles, preferably comprising a superparamagnetic iron oxide (magnetite) core provided with a coating comprising an oxidatively cleaved starch coating optionally together with a functionalized polyalkyleneoxide serves to prolong blood residence.
Reference may be made to Kresse et al. U.S. Pat. No. “57,427,767. dated Jun. 27, 1995. entitled “Nanocrystalline magnetic iron oxide” particles-method for preparation and use in medical diagnostics and therapy” wherein nanocrystalline magnetic particles consisting of genetic nanocrystalline magnetic particles consisting of genetic iron oxide core of Fe.sub.3 O.sub.4. gamma-Fe.sub.2 O.sub.3 or mixtures thereof and an envelope chemisorbed to said core, the method for preparation of these particles as well as the use thereof in medical diagnostics and/or therapy.
The magnetic particles, according to the invention, are characterized by composition of the coating material of natural or synthetic glycosaminoglycans and/or their derivatives with molecular weights of 5(X) Da to 250.000 Da. if necessary covalently cross-linked with appropriate cross-linking agents and/or modified by specific additives, oxide core of Fe.sub.3 O.sub.4. gamma-Fe.sub.2 O.sub.3 or mixtures thereof and an envelope chemisorbed to said core, the method for preparation of these particles as well as the use thereof in medical diagnostics and/or therapy. The magnetic particles, according to the invention, are characterized by composition of the coating material of natural or synthetic glycosaminoglycans and/or their derivatives with molecular weights of 500 Da to 250,000 Da, if necessary covalently cross-linked with appropriate cross-linking agents and/or modified by specific additives.
As is evident from the above mentioned recent references from US patents, superparamagnetic iron oxide (both magnetite and maghemite phases) have been used in the medical diagnostics and therapeutic uses. No evidence so far has been obtained regarding the synthesis of acicular shaped maghemite particles for the use in information storage (US patent Search), following biomimetic route.
Since the evolution of life, synthesis of nano and microsized inorganic particles is observed in nature. Under the control of a biopolymeric matrix, the in situ synthesis of these inorganic minerals exhibit a precise control over their nucleation and growth which result in precipitation of agglomeration free particles. Our teeth, bone, shells are some of the common products of biomineralisation in nature.